I'm fairly new to modding and I'm making a troll mod for someone. I have the basics down fairly well but something I would love to do is to increase the size of the Maggot Lair from 3 levels to say 6 or so. My thought would be to keep levels 1 and 3 as they are and just duplicate level 2 over and over. Something like Level 1 > Level 2a > Level 2b > Level 2c > Level 2d > Level 3 (although I would rename them Levels 1 to 6).

I'm wondering if this is something that is fairly easy and quick to do or if it is very advanced and time consuming? If it is very difficult I'll just pass on the idea for now and some day perhaps revisit it. If it is relatively easy to do, what is the bare minimum information I need to read up on to pull this off?

It is extremly easy and requires just a few minutes of practically almost only copy+paste, however, the game is limited to the number of map levels it already has. To get rid of this problem, you would have to do some code edits, but fortunately, there are public plugins called extended levels for some game versions, which are as easy to apply as copy+paste too. You can find this plugin for other game versions using forums' search tool. Be warned it is extremly unlikely anyone ever ported this to 1.14, as this game version (as everything after 1.13d) isn't of any interest to 99% of modders.

Thank you, I'll read up on the links. I had this idea in my head that the actual code to move players from one map to another was buried inside of some kind of special map files. If I choose not to use an extended levels plugin, could I simply overwrite existing levels to be able to add more Maggot Lair levels? Something like removing levels I know won't be visited (The Pit, Stony Tomb, those 3 Hell levels in act 5) and just replacing them with more Maggot Lairs?

Yes, you can replace them, but then you have to edit warps from previous levels. IE if you delete the cave level 2, you have to edit the cave level 1 so it doesnt contain an entrance to the level you deleted.